The present invention relates to the removal and recovery of radionuclides and metals from feed solutions, such as waste waters and process streams, using supported liquid membrane technology.
Liquid membranes combine extraction and stripping, which are normally carried out in two separate steps in conventional processes such as solvent extractions, into one step. A one-step liquid membrane process provides the maximum driving force for the separation of a targeted species, leading to the best clean-up and recovery of the species (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman and Hall, New York, 1992).
There are two types of liquid membranes: (1) supported liquid membranes (SLMs) and (2) emulsion liquid membranes (ELMs). In SLMs, the liquid membrane phase is the organic liquid imbedded in pores of a microporous support, e.g., microporous polypropylene hollow fibers (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman and Hall, New York, 1992). When the organic liquid contacts the microporous support, it readily wets the pores of the support, and the SLM is formed.
For the extraction of a target species from a feed solution, the organic-based SLM is placed between two aqueous solutionsxe2x80x94the feed solution and the strip solution where the SLM acts as a semi-permeable membrane for the transport of the target species from the feed solution to the strip solution. The organic liquid in the SLM is immiscible in the aqueous feed and strip streams and contains an extractant, a diluent which is generally an inert organic solvent, and sometimes a modifier.
The use of SLMs to remove radionuclides from aqueous feed solutions has been long pursued in the scientific and industrial community. Nechaev et al. (A. F. Nechaev, V. V. Projaev, V. P. Kapranchik, xe2x80x9cSupported Liquid Membranes in Radioactive Waste Treatment Processes: Recent Experience and Prospectivexe2x80x9d, in S. Slate, R. Baker, and G. Benda, eds., Proceedings of Fifth International Conference on Radioactive Waste Management and Environmental Remediation, Volume 2, American Society of Mechanical Engineers, New York, 1995) have reported on the experience and prospective of using SLMs in radioactive waste treatment processes, and the transport of uranyl ion across SLMs has been studied extensively (J. P. Shukla and S. K. Misra, xe2x80x9cUranyl Ion Transport Across Tri-n-butyl Phosphate/n-Dodecane Liquid Membranesxe2x80x9d, Proceedings of the International Symposium on Uranium Technology, Bhabha Atomic Research Centre, Bombay, India, pp. 939-946, 1991; M. A. Chaudhary, xe2x80x9cSeparation of Some Metal Ions Using Coupled Transport Supported Liquid Membranesxe2x80x9d, in H. Javed, H. Pervez, and R. Qadeer, Modern Trends in Contemporary Chemistry, Scientific Information Division PINSTECH, Islamabad, Pakistan, pp. 123 -131, 1993).
Chiarizia et al. (R. Chiarizia, E. P. Horwitz, and K. M. Hodgson, An Application of Supported Liquid Membranes for Removal of Inorganic Contaminants from Groundwater, DOE Report No. DE92006971, 1991) have reviewed and summarized the results of an investigation on the use of SLMs for the removal of uranium and some inorganic contaminants, including technetium, from the Hanford site groundwater. Chiarizia (R. Chiarizia, xe2x80x9cApplication of Supported Liquid Membranes for Removal of Nitrate, Technetium (VII) and Chromium (VI) from Groundwaterxe2x80x9d, J. Membrane Sci., 55, 39-64 (1991)) has described the separation of technetium (VII) and uranium (VI) from synthetic Hanford site groundwater using SLMs. Dozol et al. (J. F. Dozol, J. Casas, and A. Sastre, xe2x80x9cStability of Flat Sheet Supported Liquid Membranes in the Transport of Radionuclides from Reprocessing Concentrate Solutionsxe2x80x9d, J. Membrane Sci., 82, 237-246 (1993)) have studied the stability of flat sheet SLMs in the transport of radionuclides.
Recently, Dozol et al. (J. F. Dozol, N. Simon, V. Lamaare, H. Rouquette, S. Eymard, B. Tournois, D. De Marc, and R. M. Macias, xe2x80x9cA Solution for Cesium Removal from High-Salinity Acidic or Alkaline Liquid Waste: the Crown Calix[4]arenesxe2x80x9d, Sep. Sci. Technol., 34, 877-909 (1999)) have described the use of the extractant, Calix[4]arenes monocrown or biscrown, blocked in 1,3 alternative cone conformation, in SLMs for the removal of cesium from high-salinity acidic or alkaline liquid waste. Kedari et al. (C. S. Kedari, S. S. Pandit, and A. Ramanujam, xe2x80x9cSelective Permeation of Plutonium (IV) through Supported Liquid Membrane Containing 2-Ethylhexyl 2-Ethylhexyl Phosphonic Acid as Ion Carrierxe2x80x9d, J. Membrane Sci., 156, 187-196 (1999)) have studied the selective permeation of plutonium (IV) through a SLM containing 2-ethylhexyl 2-ethylhexyl phosphonic acid as the ion carrier.
One disadvantage of SLMs is their instability due mainly to loss of the membrane liquid (organic solvent, extractant, and/or modifier) into the aqueous phases on each side of the membrane (A. J. B. Kemperman, D. Bargeman, Th. Van Den Boomgaard, H. Strathmann, xe2x80x9cStability of Supported Liquid Membranes: State of the Artxe2x80x9d, Sep. Sci. Technol., 31, 2733 (1996); T. M. Dreher and G. W Stevens, xe2x80x9cInstability Mechanisms of Supported Liquid Membranesxe2x80x9d, Sep. Sci. Technol., 33, 835-853 (1998); J. F. Dozol, J. Casas, and A. Sastre, xe2x80x9cStability of Flat Sheet Supported Liquid Membranes in the Transport of Radionuclides from Reprocessing Concentrate Solutionsxe2x80x9d, J. Membrane Sci., 82, 237-246 (1993)). The prior art has attempted to solve this problem through the combined use of SLM with a module containing two set of hollow fibers, i.e., the hollow-fiber contained liquid membrane (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman and Hall, New York, 1992). In this configuration with two sets of microporous hollow-fiber membranes, one carries the aqueous feed solution, and the other carries the aqueous strip solution. The organic phase is contained between the two sets of hollow fibers by maintaining the aqueous phases at a higher pressure than the organic phase. The use of the hollow-fiber contained liquid membrane increases membrane stability, because the liquid membrane may be continuously replenished. However, this configuration is not advantageous because it requires mixing two sets of fibers to achieve a low contained liquid membrane thickness.
In ELMs, an emulsion acts as a liquid membrane for the separation of the target species from a feed solution. An ELM is created by forming a stable emulsion, such as a water-in-oil emulsion, between two immiscible phases, followed by dispersion of the emulsion into a third, continuous phase by agitation for extraction. The membrane phase is the oil phase that separates the encapsulated, internal aqueous droplets in the emulsion from the external, continuous phase (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman and Hall, New York, 1992). The species-extracting agent is contained in the membrane phase, and the stripping agent is contained in the internal aqueous droplets. Emulsions formed from these two phases are generally stabilized by use of a surfactant. The external, continuous phase is the feed solution containing the target species. The target species is extracted from the aqueous feed solution into the membrane phase and then stripped into the aqueous droplets in the emulsion. The target species can then be recovered from the internal aqueous phase by breaking the emulsion, typically via electrostatic coalescence, followed by electroplating or precipitation.
The use of ELMs to remove radionuclides from aqueous feed solutions has also been long pursued in the scientific and industrial community. The ELMs for the removal of radionuclides, including strontium, cesium, technetium, and uranium, have been described in detail (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman and Hall, New York, 1992). The extraction of strontium with the ELM technique has been investigated (I. Eroglu, R. Kalpakci, and G. Gunduz, xe2x80x9cExtraction of Strontium Ions with Emulsion Liquid Membrane Techniquexe2x80x9d, J. Membrane Sci., 80, 319-325 (1993)).
One disadvantage of ELMs is that the emulsion swells upon prolonged contact with the feed stream. This swelling causes a reduction in the stripping reagent concentration in the aqueous droplets which reduces stripping efficiency. It also results in dilution of the target species that has been concentrated in the aqueous droplets, resulting in lower separation efficiency of the membrane. The swelling further results in a reduction in membrane stability by making the membrane thinner. Finally, swelling of the emulsion increases the viscosity of the spent emulsion, making it more difficult to demulsify. A second disadvantage of ELMs is membrane rupture, resulting in leakage of the contents of the aqueous droplets into the feed stream and a concomitant reduction of separation efficiency. Raghuraman and Wiencek (B. Raghuraman and J. Wiencek, xe2x80x9cExtraction with Emulsion Liquid Membranes in a Hollow-Fiber Contactorxe2x80x9d, AIChE J., 39, 1885-1889 (1993)) have described the use of microporous hollow-fiber contactors as an alternative contacting method to direct dispersion of ELMs to minimize the membrane swelling and leakage. This is due to the fact that the hollow-fiber contactors do not have the high shear rates typically encountered with the agitators used in the direct dispersion. Additional disadvantages include the necessary process steps for making and breaking the emulsion.
Thus, there is a need in the art for an extraction process which maximizes the stability of the SLM membrane, resulting in efficient removal and recovery of radionuclides from the aqueous feed solutions.
There is also a need in the art for extractants which selectively remove a given target species from the feed stream.
The present invention relates generally to a process for the removal and recovery of target species from a feed solution using a combined SLM/strip dispersion. The invention also relates to a new family of extractants that are useful for the removal and recovery of radionuclides and metals.
In one embodiment, the present invention relates to a process for the removal and recovery of one or more radionuclides from a feed solution which comprises the following steps. First, a feed solution containing one or more radionuclides is passed on one side of the SLM embedded in a microporous support material and treated to remove the radionuclides by the use of a strip dispersion on the other side of the SLM. The strip dispersion can be formed by dispersing an aqueous strip solution in an organic liquid, for example, using a mixer. Second, the strip dispersion, or a part of the strip dispersion, is allowed to stand, resulting in separation of the dispersion into two phases: the organic liquid phase and the aqueous strip solution phase containing a concentrated radionuclide solution.
The continuous organic phase of the strip dispersion readily wets the pores of a microporous support to form a stable SLM. The process of the present invention provides a number of operational and economic advantages over the use of conventional SLMs.
In another embodiment, the present invention relates to a process for the removal and recovery of one or more metals from a feed solution which comprises the following steps. First, a feed solution containing one or more metals is passed on one side of the SLM embedded in a microporous support material and treated to remove the metals by the use of a strip dispersion on the other side of the SLM. As described above, the strip dispersion can be formed by dispersing an aqueous strip solution in an organic liquid, for example, using a mixer. The strip dispersion, or a part of the strip dispersion, is then allowed to stand, resulting in separation of the dispersion into two phases: the organic liquid phase and the aqueous strip solution phase containing a concentrated metal solution.
In yet another embodiment, the present invention relates to a family of new extractants, alkyl phenylphosphonic acids, e.g., 2-butyl-1-octyl phenylphosphonic acid (BOPPA) and 2-octyl-1-dodecyl phenylphosphonic acid (C20 ODPPA), which are useful in both conventional SLMs and the process of the present invention for the removal and recovery of radionuclide and/or metal species, also called herein the xe2x80x9ctarget species.xe2x80x9d Use of the new extractants result in improved extraction and an increased concentration of the target species in the aqueous strip solution.
Thus, it is an object of the present invention to provide an SLM process for the removal and recovery of target species which provides increased membrane stability.
It is another object of the invention to provide an SLM process having high flux.
It is yet another object of the present invention to provide an SLM process having improved recovery of the target species to provide a concentrated strip solution.
It is a further object of the invention to provide an SLM process for the removal and recovery of a target species from a feed solution which exhibits decreased operation costs and a decreased capital investment over convention SLM processes.
It is an object of the present invention to provide an SLM process for the removal and recovery of radionuclides from a feed solution.
It is another object of the present invention to provide a process for the removal and recovery of metals from a feed solution.
It is yet another object of the present invention to provide a process for the removal and recovery of strontium, cesium, technetium, uranium, boron, plutonium, cobalt, or americium from a feed solution.
It is a further object of the invention to provide a process for the removal and recovery of calcium, magnesium, and/or zinc from a feed solution.
It is an object of the invention to provide a family of new extractants, alkyl phenylphosphonic acids, for the removal of target species.
It is also an object of the invention to provide the compound 2-butyl-1-octyl phenylphosphonic acid (BOPPA) for the removal of target species.
It is another object of the invention to provide the compound 2-octyl-1-dodecyl phenylphosphonic acid (C20 ODPPA) for the removal of target species.
It is yet another object of the invention to provide a process for the removal of strontium, cesium, technetium, uranium, boron, plutonium, cobalt, or americium using an alkyl phenylphosphonic acid.
It is another object of the invention to provide a process for the removal of strontium, cesium, technetium, uranium, boron, plutonium, cobalt, or americium using 2-butyl-1-octyl phenylphosphonic acid (BOPPA).
It is another object of the invention to provide a process for the removal of strontium, cesium, technetium, uranium, boron, plutonium, cobalt, or americium using 2-octyl-1-dodecyl phenylphosphonic acid (C20 ODPPA).
It is another object of the invention to provide a process for the removal of calcium, magnesium, and/or zinc using an alkyl phenylphosphonic acid.
It is another object of the invention to provide a process for the removal of calcium, magnesium, and/or zinc using 2-butyl-1-octyl phenylphosphonic acid (BOPPA).
It is another object of the invention to provide a process for the removal of calcium, magnesium, and/or zinc using 2-octyl-1-dodecyl phenylphosphonic acid (C20 ODPPA).